The invention concerns a method for inerting the anodes of fuel cells, especially high-temperature fuel cells. In addition, the invention concerns a fuel cell system, especially a high-temperature fuel cell system, with one or more fuel cells, each of which has an anode and a cathode, and with an anode inlet for supplying an anode gas to the anodes.
Fuel cells, including, for example, molten carbonate fuel cells, have the problem that when normal operation is interrupted, e.g., during an emergency shutdown or standby operation, when no fuel gas is being supplied to the anodes, the anodes must be quickly inerted (i.e. rendered inert) to prevent them from being damaged or destroyed by oxidation. This is especially true for high-temperature fuel cells with an operating temperature of 200° C. or more. In previously known systems, it is customary to use a flushing or protective gas, which is typically nitrogen. Due to the amounts of gas that are required, it is necessary to provide a nitrogen tank specifically for this purpose, which results in additional space requirement and expense. Furthermore, the permissible shutdown time is limited by the stored supply of flushing gas.
Japanese Patent Abstract 04004570 A describes a fuel cell system, in which a standby gas that contains mainly hydrogen is used to overcome shutdown times of the fuel cell system while maintaining the operating temperature of the fuel cells. This gas is produced in a reformer by supplying the unit with the fuel gas in a smaller amount than for normal operation. At the same time, another gas, which contains carbon dioxide and hydrogen, is supplied to the cathodes at a corresponding flow rate to produce electric energy for powering a heating unit. Moreover, Japanese Patent Abstract 04324253 A describes a fuel cell system that uses a standby gas that consists of a reducing gas mixed with nitrogen to prevent oxidation of the anodes during periods when the fuel cell system is shut down.
U.S. Pat. No. 6,127,057 describes a fuel cell system in which the anode and cathode gas flow fields are flooded with cooling water, which is used as a protective fluid in place of a protective gas.
DE 40 27 655 C1 describes a method for operating an H2/O2/H2O fuel cell in which it is possible to change between a fuel cell mode of operation and an electrolysis mode of operation. During the change of the mode of operation, the cells are flushed with a stream of inert gas. This is intended to ensure that no residual gases from a preceding cycle are present in the cell.
JP 2000-277137 A describes a method in which a flushing gas that consists of carbon dioxide and nitrogen is produced by oxidation of a fuel. Therefore, it is unnecessary to store a separate supply of a standby gas. However, this method has the disadvantage of considerable additional equipment expense for producing the protective gas.